Re: [PATCH v3 33/33] block/nbd: drop connection_co

[Vladimir Sementsov-Ogievskiy]

16.04.2021 11:09, Vladimir Sementsov-Ogievskiy wrote:
> OK, that's a big rewrite of the logic.
>
> Pre-patch we have an always running coroutine - connection_co. It does
> reply receiving and reconnecting. And it leads to a lot of difficult
> and unobvious code around drained sections and context switch. We also
> abuse bs->in_flight counter which is increased for connection_co and
> temporary decreased in points where we want to allow drained section to
> begin. One of these place is in another file: in nbd_read_eof() in
> nbd/client.c.
>
> We also cancel reconnect and requests waiting for reconnect on drained
> begin which is not correct.
>
> Let's finally drop this always running coroutine and go another way:
>
> 1. reconnect_attempt() goes to nbd_co_send_request and called under
>     send_mutex
>
> 2. We do receive headers in request coroutine. But we also should
>     dispatch replies for another pending requests. So,
>     nbd_connection_entry() is turned into nbd_receive_replies(), which
>     does reply dispatching until it receive another request headers, and
>     returns when it receive the requested header.
>
> 3. All old staff around drained sections and context switch is dropped.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy<vsementsov@virtuozzo.com>

Please consider this last patch as RFC for now:

1. It is complicated, and doesn't have good documentation. Please look through 
and ask everything that is not obvious, I'll explain. Don't waste your time 
trying to understand what is not clean.

2. I also failed to image, how to split the patch into smaller simple patches.. 
Ideas are welcome.

3. It actually reverts what was done in

commit 8c517de24a8a1dcbeb54e7e12b5b0fda42a90ace
Author: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Date:   Thu Sep 3 22:02:58 2020 +0300

    block/nbd: fix drain dead-lock because of nbd reconnect-delay

and I didn't check yet, does this dead-lock still here or not. Even if it still 
here I believe that nbd driver is a wrong place to workaround this bug, but I 
should check it first at least.

--
Best regards,
Vladimir